Process for removing lead from sandblasting wastes containing paint chips

ABSTRACT

Heavy metals are efficiently removed from contaminated soil by a process which comprises leaching or washing the soil with a mild leachant solution comprised of an aqueous solution of an acid and a salt. Heavy metals are also efficiently removed from paint chips by washing with an aqueous acid. The heavy metals are recovered from the leachant be cementation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.07/930,638 filed Aug. 17, 1992, now U.S. Pat. No. 5,494,649, the entirecontents of which are incorporated by reference, which is in turn acontinuation-in-part of application Ser. No. 07/771,286, filed on Oct.3, 1991, now abandoned, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to a process for removing heavy metals from soiland paint chips.

2. Description of the Related Art

The removal of heavy metal contaminants from soils represents a majorcontemporary environmental problem. Heavy metal pollution can leave theaffected ground unusable for agricultural or residential purposes, andthe metals can eventually leach into the groundwater system and lead tomore widespread problems. While a number of soil classification orsolidification/stabilization techniques which leave the offending metalsin the soil have been developed, only removal of the metals actuallysolves the problem by removing the cause. Several attempts to removemetals from soils have been reported but none have been completelysuccessful. One such method is described in Draft Report to the EPA,Contract No. 68-03-3255, US EPA, Emergency Response Branch, Edison ,N.J., 1986 and CSIRO Aust. Div. Soils Tech. Pap. No. 41, 1979, 1-17wherein the extracted metals which were bound to clay and humicmaterials were removed with a strong complexing agent such as EDTA.However, the EDTA remained in the wetted soil causing the treated soilto fail the TCLP test of the EPA for extractable metals. Sci. TotalEnviron. 1989, 79, 253-270; Geoderma 1971, 5, 197-208; Soil Sci. Soc.Am. J. 1986, 50, 598-601; Can. J. Soil Sci. 1976, 56, 37-42; Can. J.Soil Sci. 1969, 49, 327-334; and Plant Soil 1973, 38, 605-619 teach theuse of aqueous acetic acid or ammonium acetate solutions as extractants.This method resulted in only slight leaching of metals from the soil. EP278,328 (1988); Environ. Prog. 1990, 9, 79-86; EP 377,766 (1990); andChemiker-Zeitung 1982, 106, 289-292 teach the use of strongly acidicleachant solutions such as HCl. This method leads to substantial (ca.30%) dissolution of soil components and requires extensive basificationof each HCl extract and/or the washed soil. DE 3,703,922 and DE3,705,519 teach that isolation of the metal values is often impossibledue to the metals having been precipitated as sulfides. EP 291,746 andSU 1,444,377 teach that complete separation of the soil and aqueousphases in an extraction process is difficult. J. Indian Chem. Soc.,Sect. A 1982, 21A, 444-446; Hydrometallurgy 1987, 17, 215-228; NipponKinzoku Gakkaishi, 1978, 42, 1007-1012; Hydrometallurgy, 1985, 14,171-188; J. Indian Chem. Soc. 1985, 62, 707-709; J. Anal. Chem. USSR,1983, 38, 630-635; J. Inorg. Nucl. Chem. 1970, 32, 3667-3672; Russ. J.Inorg. Chem. 1960, 5, 906; I. M. M. Bull. 1961, 70, 355 and SU 710,487teach the use of carboxylic acids such as fatty acids or Versatic™ acidsfor extraction of certain metals in standard liquid ion exchangeprocesses. "The Theory and Practice of Ion Exchange: Proceedings of anInternational Conference," Cambridge, July 1976, Streat, M. ed., Soc. ofChemical Industry: London, 1976, 38.1-38.7; Trans. Instn. Min. Metall.(Sect. C: Mineral Process. Extr. Metall.), 1974, 83, C101-104; Trans.Instn. Min. Metall. (Sect. C: Mineral Process. Extr. Metall.), 1979, 88,C31-35; "Using Solvent-Impregnated Carbon to Recover Copper fromOxidized Mill Tailings," Rep. of Invest., USDI, Bur. Mines, No. 8966,1985, pp 7 teach the use of solid-supported ion exchange reagents toremove copper from clarified solutions and from a slurry of oxidizedmine tailings. Hydrometallurgy, 1982, 8, 83-94; J. Chem. Tech.Biotechnol. 1981, 31, 345-350; Proc.--Indian Acad. Sci., Chem. Sci.1988, 100, 359-361; Proc.--Indian Acad. Sci., Chem. Sci. 1988, 100,455-457 teach the use of LIX™ 34, LIX™ 622, LIX™ 51, LIX™ 54, LIX™ 70,and Kelex™ 100 as extractants for lead from aqueous feeds, but only atmore basic pHs than with the carboxylic acids extractants mentionedabove. "ISEC '86 Int. Solv. Extract. Conf., Preprints, Vol. II," 1986,19-26; and Hydrometallurgy 1985, 14, 287-293 teach the use ofdiethylhexylphosphoric acid as a lead extractant under acidicconditions, but the S-shaped isotherm prohibits reducing the Pbconcentration in the aqueous feed down to very low levels. SeparationScience 1971, 6, 443-450 teaches the extraction of metals from aqueoussolutions by SRS-100, a high molecular weight synthetic carboxylic acid.Various soil removal processes which include the use of mineral acids,bases, surfactants, and sequestering agents are reviewed inInternational Conf. on New Frontiers for Hazardous Waste Management,Sept., 1987; The Fourth Environ. International Conf. 1983, 856-895; andEnvironmental Progress 1990, 9, 79-86. EP 402,737 teaches that heavymetals are dissolved out of sludge by strong mineral acid, and that theresulting mineral acid solution containing the heavy metals is treatablewith a heavily alkaline solution containing flocculent and foamingagent. EP 278,328 teaches a process of extracting heavy metals fromcontaminated soils by treating the soils with a number of successiveacid extractions in a counter-current fashion and precipitating theheavy metals from the recovered acid solutions. DE 3,742,235 teachesremoval of heavy metals from contaminated soils by treating the soilswith a 2-40 wt. % EDTA solution having a pH of about 6. U.S. Pat. No.4,824,576 teaches an improved process for the purification of an impureaqueous solution containing heavy metal ions which comprises passing theimpure solution through a bed of activated alumina absorbent. U.S. Pat.No. 4,746,439 teaches a process for the decontamination and removal ofat least one of silver, lead, chromium(III), zinc, or nickel ions fromaqueous waste streams by contacting the contaminated waste water at a pHof from 4 to 6 with an alkaline earth silicate solid having a surfacearea in the range of about 0.1-1000 m² /g. U.S. Pat. No. 4,883,599teaches removal of metals from aqueous solutions by passing thesolutions through an ion exchange material which consists essentially ofsulfhydrated cellulose.

Mercury contamination is a particularly difficult and insidious type ofcontamination to remediate because of the prevalence andinterconvertability of ionic and elemental forms of mercury within asingle site by natural weathering action as well as biological redoxmechanisms. M. Meltzer, et al. [Pollution Technology Reviews, No. 196,Noyes Data Corp, Park Ridge, N.J., 1990, p. 373] teaches that watersoluble and insoluble ionic mercury compounds are bioavailable forreduction to mercury metal by bacterial action, including the highlyinsoluble mercuric sulfide. The conversion of elemental mercury intowater soluble ionic forms is also biologically possible as well asconversion into volatile dimethyl mercury. All forms and compounds ofmercury are toxic including elemental mercury. ["The Merck Index,"11^(th) Ed., Merck & Co., Inc., Rahway, N.J., 1989, p 5805; P. C.Bidstrup, "Toxicity of Mercury and its Compounds," Elsevier, Amsterdam,1964; L. Magos, Br. Med. Bull. 1975, 31, 241-5] The toxicity of theelemental form can not only be experienced by direct ingestion, but alsoby inhalation due to the relatively high vapor pressure of mercury,2×10⁻³ mm (25° C.). The vapor pressure of mercury alone results in aconcentration 200 times higher than the maximum allowed contentration,0.01 ppm. Long exposure to mercury also produces a cummulative effect. Anumber of processes have been disclosed which claim to oxidativelydissolve elemental mercury and allow the reclamation of the metal. Theseprocesses all have disadvantages. Hot nitric acid solutions are known tooxidatively dissolve mercury while reducing nitrate to nitrogen oxides.[DE 3812986, 1989; DE 3703922, 1988] At room temperature an excess ofnitric acid is required and extended periods of time. Under fieldConditions this will produce substantial quantities of volatile andregulated nitrogen oxides which will require scrubbing before venting tothe atmosphere. Additionally, nitrate contamination of groundwaterremains a concern because of the large excess of nitric acid required.Hydrogen peroxide is also claimed to oxidize mercury metal to mercuricions. [USSR 431115, 1974] Catalysis by ferric or iodide ion is alsoreported. [EP 88-118930, 1988] Hypochlorite in combination with hydrogenperoxide is also claimed. [JP Kokai 63156586, 1988] Hydrogen peroxidesuffers the drawback in soil remediation use of being decomposed rapidlyand irreversibly with manganese dioxide, a ubiquitous soil constitutent.[EP 88-118930, 1988] This reaction produces useless oxygen gas andwater. Peracetic acid at 80° C. has been used to produce mercuricacetate from mercury metal. Hydrogen peroxide in the presence of aceticacid was also successful. [U.S. Pat. No. 2,873,289, 1959]. Hypochloriteoxidation of metallic mercury is known. Control of the pH and chlorideion concentration is required to ensure solubility of the mercuric ion.[U.S. Pat. No. 3,476,552, 1969; Eng. Mining J. 1970, 171, 107-9].Halogens, including chlorine, bromine, and chlorine with a bromide ioncatalyst, are known to dissolve mercury metal and mercuric sulfide.[U.S. Pat. No. 5013358, 1991; U.S. Pat. No. 3,424,552, 1969; Chem.Abstr. 1990, 114, 232369c; Chem. Abstr. 1988, 109, 173955n] In soilremediation applications, this requires the use of highly toxic,volatile and corrosive materials in highly populated areas which makesthis option less desirable than its use in remote mining locations.Additionally, halogens will react rapidly with organic humic matter inthe soil to produce substantial amounts of chlorinated material,including chlorinated phenols. These chlorinated species would presentdifficulties with regulatory agencies. Cyanide solutions are known todissolve mercury metal to produce soluble mercuricyanide complexes. Thedanger of using cyanide solutions in populated areas limits the utilityof this approach. In addition, thermal methods of removing mercury fromcontaminated soil by distilling the metal are known. These suffer thedrawback of the high cost of heating soil to approximately 600° C. [DE3928427, 1991; DE 3706684, 1987; "Treatment Technologies," US EPA,Office of Solid Waste, Government Inst., Inc., 1990, p 17-1]. Methodsfor treating metallic mercury and leaving it in the soil are also known.The long term acceptability of such practice is unknown. One example,ferric chloride oxidation of finely divided mercury metal, produces athin layer of mercurous/mercuric chlorides which were reacted with asulfide salt to produce a mercury particle reportedly coated with alayer of mercuric sulfide. This material could be further stabilized byknown solidification techniques. [J P Kokai 81 07697, 1981; DE 3814684,1989]. Amalgamation of metallic mercury with aluminum or iron depositedonto carbon is reported. The amalgam was claimed to be nonhazardous. [JPKokai 73 75354, 1973; EP 342898, 1989]. Extraction of mercuric ions fromthe loaded leachate can be accomplished by a number of processes. [J.Ortega, J. Gutierrez, in "Recovery of Valuable Products from Wastes,"Ortega et al. ed.] Recovery of mercury from concentrated solutions isalso known by electrochemical reduction as disclosed in U.S. Pat. No.3,647,958 and D. Bender, F. Riordan, "Metal Bearing Waste Streams,Minimizing, Recycling and Treatment," M. Meltzer, et al., ed., NoyesData Corp., Pollution Technology Review No. 196, Park Ridge, N.J., 1990,p. 298], and by reduction by iron as disclosed in U.S. Pat. No.5,013,358; by reduction by sodium borohydride [Morton Thiokiol, Inc.,Ven Met Brochure, Ventron Division, 1984], and precipitation withsulfide is known [N. H. Feigenbaum, Ind. Wastes (Chicago), 1977, 23,324].

SUMMARY OF THE INVENTION

It is an object of the present invention to efficiently remove heavymetals from contaminated soil or paint chips. The present inventionachieves this objective by simple and concise processes. One processaccording to the invention removes heavy metals other than mercuryincluding ionic forms thereof from the soil and comprises leaching orwashing soil with a mild leachant. The leachant solution is comprised ofan aqueous solution of an acid and a salt. The anion of the acid forms awater-soluble salt with all of the heavy metals which contaminate thesoil. The salt component is comprised of at least one alkali metal,alkaline earth metal, or ammonium salt having one or more anions whichalso form a water-soluble salt with the heavy metals leached from thecontaminated soil. The process can be modified by adding a second step.After the leachant solution has contacted the contaminated soil, aliquid phase containing dissolved heavy metal ions is formed and isseparated from the solids. The clarified liquid phase which remainsafter the solids have separated is treated with an extractant or aprecipitant to remove the heavy metal ions. Where the extractant isadsorbed on an inert solid support or is a solid ion exchange material,the process can also be modified by accomplishing simultaneously boththe leaching and extraction steps described above. Where the heavy metalto be removed from the soil is mercury, another process according to theinvention comprises mixing soil and a liquid leachant composition whichis an aqueous solution of (i) an acid whose anion forms a water-solublesalt with mercury; (ii) an alkali metal, alkaline earth metal, or anammonium salt having one or more anions which form water-soluble saltswith mercury, and (iii) an oxidant selected from the group consisting ofa persulfate salt, nitric acid, and a halogen in such a manner as todisperse at least part of said soil in the leachant to form a liquidphase containing dispersed soil solids and for a period of timesufficient to transfer at least a portion of the mercury from thedispersed solids to a soluble mercury species in the liquid phase.Another process according to the invention is directed at removing lead,tin, or copper or mixtures thereof from paint chips which have beenremoved from copper, tin, or lead surfaces and/or paint chips which arecomprised of paint which contains copper, tin, or lead or combinationsthereof. Such process comprises contacting the paint chips with anaqueous acid leachant selected from the group consisting of formic acid,acetic acid, propionic acid, methanesulfonic acid, hydrochloric acid,nitric acid, and sulfuric acid for a period of time sufficient totransfer at least a portion of one or more of copper, tin, or lead fromthe paint chips to one or more soluble species in the leachant. Thepresent invention is also directed toward a process for the completeremediation of mercury from contaminated soil which comprises the stepsof: (1) mixing the soil and a liquid leachant composition which is anaqueous solution of (i) an acid whose anion forms a water-soluble saltwith mercury; (ii) an alkali metal, alkaline earth metal, or an ammoniumsalt having one or more anions which form water-soluble salts withmercury, and (iii) an oxidant selected from the group consisting of apersulfate salt, nitric acid, and a halogen in such a manner as todisperse at least part of said soil in the leachant to form a liquidphase containing dispersed soil solids and for a period of timesufficient to transfer at least a portion of the mercury from thedispersed solids to a soluble mercury species in the liquid phase; (2)contacting the liquid phase with a metal such as aluminum, iron, ormagnesium to remove the soluble mercury species from liquid phase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram for the simplest aspect of the processaccording to the invention which comprises a leaching step preceded byan optional classification step.

FIG. 1a is a process flow diagram for the first modification of theprocess according to the invention wherein the leaching step iscontinued after the optional classification step, the soil or soil finesare separated from the leachant solution, and the heavy metals areextracted from the clarified leachant solution.

FIG. 1b is a process flow diagram for the second modification of theprocess according to the invention wherein the leaching step iscontinued after the classification step and combined with the extractionstep wherein the extractant is a solid. The solid extractant isseparated prior to the separation of the soil fines from the leachantsolution. The solid extractant is stripped of extracted heavy metalions, and the separated leachant solution is recycled for furtherleaching of contaminated soil.

FIG. 1c is a process flow diagram for the operation of the secondmodification of the process according to the invention wherein theprocess of FIG. 1b is varied by recycling both the soil fines-leachantslurry and stripped extractant to leaching-extraction until anacceptable level of heavy metal removal has been attained.

FIG. 2 is a graph of the leaching effectiveness of various acetic acidsolutions.

FIG. 3 is a graph of the leaching effectiveness of various hydrochloricacid solutions.

FIG. 4 is a graphic illustration of the effectiveness of various aceticacid/ammonium acetate solutions as leachants for lead.

FIG. 5 is a graphic illustration of the relative effectiveness ofvarious solutions as leachants for lead.

FIG. 6 is a graphic illustration of the effectiveness of 5% aceticacid/2.5% acetate salt solutions in leaching lead from various forms oflead in clay matrices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Other than in the operating examples, or where otherwise indicated, allnumbers expressing quantities of ingredients or reaction conditions usedherein are to be understood as modified in all instances by the term"about". For purposes of this invention, a heavy metal is a metal otherthan mercury and includes a transition metal, a lanthanide, an actinide,thallium, lead, bismuth or tin. The term soil means the upper layer orlayers of the earth and includes sediment or silt below a body of water.Contaminated soil is soil that contains unacceptably high levels ofheavy metals. Additionally, organic compounds deleterious to the healthand safety of plants, animals and humans such as hydrocarbons,chlorinated hydrocarbons, poly-chlorinated biphenyls, dioxin, highconcentrations of herbicides and/or insecticides, and the like may bepresent. Unacceptably high levels of contaminants are those levels whichare higher than threshold levels set by governmental regulatoryagencies. The term paint chip refers to a particle of solid paint filmwhich had been applied to a surface as a liquid and subsequently curedto become a protective coating and then removed by some mechanical meanssuch as by scraping, sandblasting, or blasting with plastic shot balls.The paint chips may or may not be mixed with other solid particles suchas sandblasting or plastic shot residues and the like. An acid whoseanion forms a water-soluble salt with a heavy metal according to theinvention is any acid the anion or anions of which form a salt with aheavy metal as defined herein having a solubility in water which isequal to or greater than 10⁻⁴ moles per liter. Examples of such acidsinclude but are not limited to formic acid, acetic acid, citric acid,propionic acid, methanesulfonic acid, hydrochloric acid, nitric acid,and sulfuric acid. A salt whose anion forms a water-soluble salt with aheavy metal according to the invention is any alkali metal, alkalineearth metal, or an ammonium salt having one or more anions which formwater-soluble salts with a heavy metal as defined herein. An ammoniumsalt according to the invention is a salt having an ammonium ion of theformula R₁ R₂ R₃ NH⁺ wherein each of R₁, R₂, and R₃ is independentlyhydrogen, methyl, or ethyl. Examples of such salts include but are notlimited to those salts the cations of which are NH₄ ⁺,Ca²⁺,Mg²⁺,Na⁺,K⁺,or Li⁺ and the anions are acetate, Cl⁻,NO₃ ⁻, or HSO₄ ⁻ /SO₄ ²⁻.

In its simplest aspect, the process according to the invention forremoving heavy metals other than mercury from soil can be carried out bymixing contaminated soil with a leachant which is an aqueous solutioncomprised of: (a) an acid whose anion forms a water-soluble salt withthe heavy metal ions and (b) at least one alkali metal, alkaline earthmetal, or ammonium salt having one or more anions which formwater-soluble salts with the heavy metal ions. The contaminated soil andleachant are mixed together in any convenient manner such as by stirringthe contaminated soil and the leachant solution together in a container.The ratio of soil to leachant will typically be 1 part by weight soil tofrom 2 to 10 parts by weight leachant. Preferably, the ratio of soil toleachant is 1 part by weight soil to from 2 to 5 parts by weightleachant. The soil will be contacted for a time sufficient to transferat least a portion of the heavy metals to the leachant. The soil willtypically be in contact with the leachant for up to 60 minutes. Soilcontaining elemental heavy metals may require longer leaching times toachieve dissolution. Optionally, at the start of the contacting period,a coarse solid phase of denser or larger soil particles, which isgenerally leached more rapidly than the soil fines, is separated byknown size separation techniques, such as wet classification,centrifugal separation, hydrocyclone separation, or wet screening. Suchtechniques are further described in "Solids-Liquid Separation," Chem.Eng., 1955, 62, 175-238, the entire contents of which is incorporatedherein by reference. This operation classifies, or size segregates, thesoil so that the remaining aqueous slurry contains only fine soil, siltand clay particles having a diameter of generally less than 0.02 mm.Since the majority of the heavy metal contaminants are associated withthe soil fines, more extensive leaching of the soil fines is normallyrequired than for the coarse soil components. In this way, the majorityof the soil containing the coarse constituents can be quickly freed ofcontaminants and disposed of, and the remaining soil fines can befurther leached to more completely dissolve the contained heavy metals.The process according to the invention can be carried out at anytemperature and will typically be carried out at ambient temperature.The minimum acid concentration in the leachant solution must be suchthat a pH of 7 or below is maintained during the leaching period inwhich the soil and/or the soil fines are contacted by the leachantsolution. The maintenance of the pH of the leachant solution can beaccomplished by intermittent addition of the acid to the solution or bycontinuous addition of the acid to the solution. The method of additionis not critical as long as the pH is maintained below 7. While the acidconcentration can be any concentration required to maintain the pH at orbelow 7, the maximum acid concentration in the leachant solution willtypically be about 2 moles per liter. In cases where the acid is a weakacid such as acetic acid the preferred concentration is in the range of1-10 wt % and more preferably, in the range of 1-5 wt %. In cases wherethe acid is a strong acid such as hydrochloric acid, the preferredconcentration is in the range of 0.01-1 molar and more preferably in therange of 0.05-0.5 molar. The minimum salt concentration in the leachantsolution will be typically 0.05% while the maximum salt concentration inthe leachant solution will be about 20% by weight. A preferred saltconcentration is within the range of 1-20 wt %. A more preferred saltconcentration is within the range of 2-10%.

It is contemplated in this invention that the soil can be contacted withleachant multiple times to obtain sufficient removal of the heavymetals. Liquid-solid separation of partially leached soil gives a liquidphase containing a portion of the total of the heavy metal ionsoriginally present in the soil and a solid phase which can be recycledfor further leaching with fresh or regenerated leachant. Contacting andseparating soil and leachant can be done continuously or in a batchmode.

FIG. 1 is a process flow diagram for the simplest aspect of the processaccording to the invention, in which soil is agitated with the leachant.The total time required to leach the heavy metals from the soil or soilfines and will typically take about 60 minutes or less. Optionally, theleaching step may be preceded by a classification step in the presenceof the leachant to wash and remove a coarse fraction of soil. After theleaching step, the soil fines can optionally be separated from theleachant in the solid/liquid separation step. The soil or clay/humussoil fines may be disposed of if they are sufficiently free of heavymetals or they may be subjected to further leaching.

FIG. 2 illustrates the high degree of effectiveness of a mild (pH=4)leachant, namely acetic acid plus ammonium or calcium acetate forleaching lead oxide from a clay matrix. FIG. 3 illustrates the degree ofeffectiveness of a lower pH leachant, 0.1M hydrochloric acid plusammonium or calcium chloride for leaching lead oxide from a clay matrix.The novel leachants are substantially and surprisingly more effectivethan acetic acid or hydrochloric acid alone, ammonium or calcium acetatealone or with other additives as shown in FIG. 3 and FIG. 4. The novelacetic acid based leachants clearly leach more lead from contaminatedsoil faster than even the best hydrochloric acid based leachants asshown by curves repeated in FIG. 5. The acetic acid based leachants arealso especially useful because they dissolve Pb metal at pH=4-7 as shownin FIG. 6. Thus, the repetitive use of the acetic acid/acetate-basedleachants is very efficient and effective at removing lead from soilconstituents.

The process can be modified by adding subsequent liquid-solid separationand heavy metal recovery steps. Accordingly, in this first modificationof the process according to the invention, the leaching step asdescribed above is followed by the steps of separating the liquidleachant from the solid soil particles by any liquid-solid separationtechnique such as settling, filtering or centrifuging, and recoveringthe dissolved heavy metal from the separated liquid leachant bycontacting with a water-insoluble extractant or a precipitant in orderto remove the heavy metal ions.

When a precipitant is used to remove the heavy metal ions the choice ofthe precipitant will depend upon the nature of the heavy metalcontaminants and can be readily made by those skilled in the art. Theuse of such precipitants for aqueous waste solutions is described in"Treatment Technologies," Office of Solid Waste, U.S. EPA, Govt. Inst.,Inc., 1990. Typically, the precipitants will be calcium hydroxide,sodium hydroxide, sodium carbonate, sodium sulfide or sodiumdimethyldithiocarbamate. Such precipitants will typically be used instoichiometric excess to the heavy metals in the leachant solution whenthe leachant is not recycled back to the leaching step.

The water insoluble extractant can be: (a) a complexing agent for theheavy metal ions which is either dissolved in a water-immisciblesolvent, adsorbed on an inert, solid support or adsorbed on a membranesupport; (b) an ion exchange resin or an ion selective membrane; or (c)a solid adsorbent material. Examples of complexing agents which can beused as extractants include but are not limited to: carboxylic acids,such as Versatic™ 911 (Shell), Neodecanoic™ acid (Exxon) oralpha-bromolauric acid; phosphorus acids, such asdi-2-ethylhexylphosphoric acid, Cyanex™ 302 (Cyanamid,bis(2,2,4,4-trimethylpentyl) monothiophosphinic acid); phosphorusesters, such as tributyl phosphate or dibutyl butylphosphonate;phosphine oxides, such as trioctylphosphine oxide; phenolic oximes, suchas LIX™ 84 (Henkel, 2-hydroxy-5-nonylacetophenone oxime) or LIX™ 860(Henkel, 5-dodecylsalicylaldehyde oxime); beta-diketones, such as LIX™54 (Henkel, 1-phenyldecan-1,3-dione) or LIX™ 51 (Henkel,1-(p-dodecylphenyl)-4,4,4-trifluorobutan-1,3-dione; amines, such astrilaurylamine or trioctylamine; quaternary ammonium salts, such asAliquat™ 336 (Henkel, tricaprylammonium chloride); and8-hydroxyquinolines, such as LIX™ 26 (Henkel,7-dodecenyl-8-hydroxyquinoline). The preferred extractants aresaturated, linear and branched carboxylic acids having from 7 to about30 carbon atoms. Particularly preferred carboxylic acids areNeodecanoic™ acid (Exxon) and Versatic™ acids (Shell).

Examples of water immiscible solvents for dissolving the extractants arethe aliphatic and aromatic hydrocarbons having flash points of 150° F.and higher and solubilities in water of less than 0.1% by weight. Thesesolvents are also essentially non-toxic and chemically inert and thecosts thereof are currently within practical ranges. Representativecommercially available solvents include but are not limited to: Kermac470B (an aliphatic kerosene available from Kerr-McGee-Flash Point 175°F.), Chevron Ion Exchange Solvent (available from Standard Oil ofCalifornia--Flash Point 195° F.), Escaid 100 and 110 (available fromExxon-Europe--Flash Point of ≈180° F.), Norpar 12 (available fromExxon-U.S.A.--Flash Point 160° F.), Conoco C-1214 L (available fromConoco--Flash Point 160° F.), Aromatic 150 (an aromatic keroseneavailable from Exxon-U.S.A.--Flash Point 150° F.) and various otherkerosenes and petroleum fractions available from other oil companies.The use of said water immiscible solvents to recover the heavy metal bysolvent extraction is especially advantageous when the heavy metalextraction process is followed by bioremediation.

Solid supports for insoluble extractants can be nonfunctionalized resinbeads. Examples of nonfunctionalized resin beads are the Amberlite™ XADseries, XAD-2, XAD-4, XAD-7 produced by Rohm and Haas.

Membrane supports are microporous polymeric films which may be imbibedwith liquid extractants to selectively allow heavy metal ions to passthrough. Such membranes are described in "Interphase Transfer Kineticsfrom Transport Measurements Through Supported Liquid Membranes", "ISEC'86 Int. Solv. Extract. Conf., Preprints, Vol. II," 1986, 255-262, theentire contents of which are incorporated herein by reference.

Extractants with a variety of functionalities can be adsorbed onto asolid support and used to extract heavy metals. The appropriatecombination of extractant and solid support will depend on the metal tobe extracted and the conditions desired for extraction and stripping andcan be determined by one of ordinary skill in the art. One method ofdetermining the appropriate combination for a particular applicationwould be to select an extractant which shows the desired characteristicsin liquid-liquid extraction, imbibing that extractant onto a candidatesolid support, and evaluating its extraction and stripping performancewith a representative leach solution containing the heavy metal to beextracted.

The water insoluble extractant can be an ion exchange resin which willexchange another ion or ions for the heavy metal ions. The functionalitywhich can be used on the ion exchange resin will depend on the heavymetal to be extracted, the presence of other ions, and the pH of theaqueous phase. Examples of representative functionalities includesulfonic acid, carboxylic acid, tertiary amine, quaternary ammonium, andchelating groups such as amino-diacetic acid. Such ion exchange resinsare described in "Ion Exchangers, Properties and Applications," K.Dorfner, Ann Arbor Sci. Publ., Ann Arbor, Mich., 1972 and "IonExchange," F. Helfferich, McGraw-Hill, N.Y., 1962 and their use is knownto those skilled in the art.

Ion selective membranes are described in "Encyclopedia of ChemicalTechnology", Kirk-Othmer, 8, 727-9, the entire contents of which areincorporated herein by reference.

The solid adsorbent material can be coal, carbon, or crumb rubber. Coalis particulate bituminous, sub-bituminous, lignite or peat having aparticle size of greater than about 16 mesh. Carbon is particulatecarbon or activated carbon having a particle size greater than about 16mesh (1.00 mm). Commercially available examples having a mesh size of6×16 are Calgon type GRC-22™, GRC-20™, Norit-RO™, and Norit-C™. The useof coal and carbon for heavy metal extraction is described in WaterResearch 1982, 16, 1113-1118; Water Research 1986, 20, 449-452; andWater Research 1982, 16, 1357-1366, the entire contents of which areincorporated herein by reference. Crumb rubber is particulate rubberfrom recycled tires having a particle size of greater than about 16mesh. The use of crumb rubber as a heavy metal extractant is describedin Water Research 1981, 15, 275-282, the entire contents of which isincorporated herein by reference.

FIG. 1a is a process flow diagram of the first modification of theprocess according to the invention. In the first step, soil is mixedwith a leachant. Optionally, a prior size classification can beconducted in which the coarse particles which are removed aresubstantially free of heavy metal ions and are returned to the earth asclean fill. The soil or soil fines, which remain suspended as a slurryin step 2, and the leachant are further mixed together with the leachantin step 3 until the heavy metal ions are transferred from the soil orsoil fines to the leachant as indicated by an analysis of a sample ofthe fines such as by atomic absorption spectroscopy. The time requiredto leach the heavy metals from the soil fines will typically be about 60minutes or less. The soil or soil fines are then separated from theleachant in the solid/liquid separation step 4. The soil or clay/humussoil fines may be disposed of if they are sufficiently free of heavymetals or they may be returned to step 3 for further leaching. Theliquid phase which results from the solid/liquid separation stepcontains dissolved heavy metal ions. This heavy metal-containingleachant is then extracted in step 5 with a solid or liquid extractantto remove the dissolved heavy metal ions. The heavy metal-loadedextractant is stripped by washing with strong acid to remove the heavymetals. The stripped extractant is then reused in the extraction stepand the regenerated leachant is returned to the classification orleaching steps. Alternatively, the soil may be preclassified by anothertechnique and only the soil fines leached, or the soil may be leachedwithout classification.

The amount of extractant and mode of extraction are chosen so that theion exchange capacity of the extractant or the adsorption capacity ofthe adsorbent for heavy metals is greater than the amount of heavymetals to be extracted. Thus in a single, batchwise contact of heavymetal containing leachant and extractant, an extraction or adsorptionagent with an excess capacity for extraction or adsorption would bepresent.

A preferred embodiment of the process according to the inventioncomprises mixing contaminated soil comprised of coarse and fineparticles with an aqueous solution comprised of (a) an acid whose anionforms a water-soluble salt with the heavy metal ions and (b) at leastone alkali metal, alkaline earth metal, or ammonium salt having one ormore anions which form water-soluble salts with the heavy metal ions toproduce a liquid slurry phase and a coarse solid soil phase. The coarsesolid phase is removed by classification, and the liquid slurry phasecontinues to be leached. The liquid slurry phase is then subjected to aliquid-solid separation to produce a clarified liquid phase containingat least a portion of the heavy metals and a solid phase containing thesoil fines. The clarified liquid phase is then contacted with awater-insoluble extractant dissolved in a water-immiscible solvent, awater-insoluble extractant adsorbed on an inert solid support, anadsorbent, or an ion exchange resin to remove the heavy metals from theclarified liquid to produce regenerated leachant which is then returnedfor further leaching of contaminated soil. The solid phase containingthe soil fines may be returned to further leaching with fresh orregenerated leachant. The leaching and liquid-solid separation of thesoil fines is repeated until the heavy metal contaminants in the soilfines have been reduced to the desired levels such as those prescribedby state and federal regulatory agencies.

In another preferred embodiment, the aqueous leachant is comprised ofacetic acid and an acetate salt of an alkaline earth metal or anammonium ion as defined herein, and the extractant is activated carbonwhich removes the heavy metal ions from the liquid phase. The heavymetal ions are adsorbed by the carbon and can subsequently be removedfrom the carbon complex by washing with a strong acid. The heavy metalsare then present in concentrated form in an acid solution for furtherisolation or processing.

In a second modification of the process according to the invention, bothsteps of leaching and contacting with a solid extractant as set forthherein are accomplished simultaneously. Since contacting the soil onetime with a leachant may remove less than all of the heavy metalspresent in the soil sample, repeated contacts of soil with leachant willoften be needed to sufficiently remove the metal ions. Therefore, thissecond modification of the process according to the invention comprisescarrying out the leaching and extracting steps simultaneously by mixingcontaminated soil fines simultaneously with a leachant component and awater-insoluble solid extractant component as disclosed herein. In thissecond modification the soil must be classified to the extent that theremaining soil fines are separable from the solid extractant. The sizedifference between the soil particles and extractant particles should beat least a factor of two and preferably at least a factor of ten. Theleachant component is an aqueous solution comprised of an acid whoseanion forms a water-soluble salt with said heavy metal ions and at leastone alkali metal, alkaline earth metal, or ammonium salt having one ormore anions which form water-soluble salts with said heavy metal ions.The extractant component is either a water-insoluble extractant adsorbedon an inert, solid adsorbent material or adsorbed on a membrane support;an ion selective membrane; a solid adsorbent; or an ion exchange resin.A liquid slurry phase is thus obtained which contains the leachantcomponent, soil fines, dissolved heavy metal ions, and the extractantcomponent now containing at least a portion of the heavy metal incomplexed form. This extractant component is separated from the liquidslurry phase to remove the extractant component loaded with the heavymetal ion complex. The loaded extractant component is then contactedwith a stripping solution to remove the heavy metal in concentratedform, and regenerate the extractant for further contact with liquidslurry phase.

The solid extraction component can be removed by passing the liquidslurry phase through a screen. A liquid slurry stream containing soilfines and heavy metals not removed in the first contact passes throughthe screen. This liquid slurry phase stream is then recycled backthrough the leaching process with fresh or regenerated solid extractantcomponent for a number of cycles sufficient to reduce the heavy metalconcentration in the soil fines to the desired levels such as thoseprescribed by state and federal regulatory agencies. An advantage ofthis modification is the avoidance of expensive and time-consumingrepetitive liquid-solid separation to produce clarified aqueous solutionfollowing each successive contact of leachant with the soil. Thismodification of the process according to the invention can also becarried out in a continuous manner by continuously removing the loadedextractant component from the liquid slurry phase and adding fresh orregenerated extractant component so that the continuous leaching of thesoil fines allows high metal removal. The liquid slurry phase and thesolid extractant may also be continuously advanced counter-currently toeach other. Thus the heavy metals dissolve into the leachant and aredirectly complexed by the extractant component. This prevents buildup ofhigh heavy metal concentrations in the leachant, and displaces thedissolution equilibrium toward completion, giving more rapid andcomplete removal of heavy metals from the soil. The resulting lowconcentration of heavy metals in solution permits the use of leachantacids and salts that might not otherwise have adequate solubility withthe heavy metal, such as lead sulfate, but will give sufficientsolubility for simultaneous leaching and extraction.

The amount of solid extractant and mode of extraction are chosen so thatthe ion exchange capacity of the extractant or the adsorption capacityof the adsorbent for heavy metals is greater than the amount of heavymetals to be extracted. Thus in a single, batchwise contact ofcontaminated soil and extractant, an extraction or adsorption agent withan excess capacity for extraction or adsorption would be present.Alternatively, when the process is conducted continuously or withmultiple contacts of the soil with extractant, the heavy metalconcentration may initially be in excess of the capacity of theextractant present, but further contacting of the soil with fresh orregenerated extractant provides the excess capacity needed to removesubstantially all of the heavy metal ions from the leachant solution.

FIG. 1b is a process flow diagram for one aspect of the secondmodification of the process according to the invention wherein theextractant is a solid as set forth herein. In the first step the soil isclassified with the leachant, in the second step the soil fines remainsuspended as a slurry in the leachant and are further mixed togetherwith solid extractant in step 4 to transfer the heavy metals ions fromthe soil fines via the leachant to the solid extractant. The total timefor steps 1 and 4 is the time required to leach the metals from the soilfines and will typically take about 60 minutes or less. The solidextractant is then removed in step 5 by passing the leachant-soilfines-extractant slurry through a screen and recovering a regeneratedleachant/soil fines slurry from which the soil fines are then removed instep 6 to produce a regenerated leachant stream for recycle to theclassification/leaching step. The heavy metal-loaded solid extractant isstripped by washing with strong acid to remove the heavy metals. Thestripped extractant in step 3 is then reused in the extraction step 4.Alternatively, the contaminated soil may be preclassified by anothertechnique and only the soil fines leached in step 4.

A process flow diagram for another aspect of the second modification ofthe process according to the invention is depicted in FIG. 1c. Theaspect is similar to that depicted in FIG. 1b except that theregenerated leachant containing the soil fines and formed after step 5is recycled back into step 4. Thus, the soil fines are continuously andrepeatedly contacted by the leachant and solid extractant until theheavy metal ion content reaches an acceptable value. This continuousprocess is depicted by the solid arrows. After the heavy metal ionconcentration reaches an acceptable level, the process continues throughthe hatched arrows wherein the cleaned soil fines are separated from theleachant and returned to the earth as fill. The heavy metal-freeregenerated leachant can be returned to step 1 of the process.

A preferred embodiment of the second modification of the processaccording to the invention is comprised of a leaching step whichcomprises contacting contaminated soil with a leachant comprised of anacid whose anion forms a water-soluble salt with said heavy metal ionsand at least one alkali metal, alkaline earth metal, or ammonium salthaving one or more anions which form water-soluble salts with said heavymetal ions. The coarse solid phase of denser or larger soil particles isthen separated by known size separation techniques. This classifies, orsize segregates, the soil so that the remaining first liquid slurrycontains only fine soil particles, generally less than 0.02 mm indiameter. A carbon is added to produce a first liquid slurry phase and acoarse solid extractant phase. After sufficient time to transfer atleast a portion of the heavy metals to the solid extractant, the firstliquid slurry phase is passed through a screen having a mesh size offrom about 80 (0,177 mm) to about 5 (4.00 mm) to remove the solidextractant phase which contains the heavy metal ion-carbon complex. Asecond liquid slurry phase stream containing soil fines and heavy metalions not removed in the first contact results from this separation stepand is recycled back to the leaching step. The solid extractant phasewhich remains on the screen is removed from the screen and the heavymetal ions are stripped by washing with a strong acid such as 3Mhydrochloric acid. The stripped extractant-solid is recycled back to theleaching step. The heavy metals are then present in an acid solution inconcentrated form for further isolation or processing to recover theheavy metals. After a sufficient number of leaching cycles, the treatedsolid soil may be separated from the leachant and returned to the earth.The processes according to the invention are particularly effective inremoving lead, zinc, cadmium, and bismuth from contaminated soil.

The modified processes according to the invention can be even furtheraltered by adding a bioremediation step at the end of the process. Inother words, soil treated by the modified processes according to theinvention in which the leaching step is followed by or carried outsimultaneously with the extracting step or continuously with theextracting step can be freed of unwanted organic matter by subjectingthe treated soil to bioremediation. Bioremediation is a process in whichmicroorganisms are used to degrade organic contaminants to harmlessby-products. An organic contaminant is any organic matter whose presencein soil renders the soil unfit for plant, animal or human contact oruse. Examples of organic contaminants include hydrocarbon oils,gasoline, and heavier petroleum hydrocarbons; aromatic compounds such asbenzene, toluene, xylene, and polynuclear aromatics; chlorinatedhydrocarbons such as chlorinated benzenes, pentachlorophenol, andpoly-chlorinated biphenyls; dioxin, herbicides and/or insecticides, andthe like. Bioremediation may be accomplished by any method known tothose skilled in the art such as by land treatment, bioreactors, andin-situ treatment. The use of bioremediation as a means of cleaningwastes of hazardous organic materials is described in the Aug. 26, 1991edition of Chemical and Engineering News, pages 23-34, the entirecontents of which are incorporated herein by reference. Bioremediationis often hindered or even prevented when heavy metals are present due totheir high toxicity. Thus, removal of heavy metals may be a necessaryfirst step to allow bioremediation to take place. The use of acetic acidand acetate salt as the leachant is particularly preferred when heavymetal recovery is to be followed by bioremediation, since it leaves aresidue of an only mildly acidic solution in the soil, and the residualacetate can act as a nutrient to the remediating microorganisms.

In its simplest aspect, the process according to the invention whichremoves mercury from soil containing mercury can be carried out bymixing mercury-containing soil with a liquid leachant composition whichis an aqueous solution of (i) an acid whose anion forms a water-solublesalt with mercury; (ii) an alkali metal, alkaline earth metal, or anammonium salt having one or more anions which form water-soluble saltswith mercury, and (iii) an oxidant selected from the group consisting ofa persulfate salt, nitric acid, and a halogen in such a manner as todisperse at least part of said soil in the leachant to form a liquidphase containing dispersed soil solids and for a period of timesufficient to transfer at least a portion of the mercury from thedispersed solids to a soluble mercury species in the liquid phase. Anacid whose anion forms a water-soluble salt with mercury according tothe invention is any acid the anion or anions of which form a salt withmercury which has a solubility in water which is equal to or greaterthan 10⁻⁴ moles per liter. Examples of such acids include but are notlimited to formic acid, acetic acid, propionic acid, methanesulfonicacid, hydrochloric acid, nitric acid, and sulfuric acid. A salt whoseanion forms a water-soluble salt with mercury according to the inventionis any alkali metal, alkaline earth metal, or an ammonium salt havingone or more anions which form water-soluble salts with mercury asdefined herein. An ammonium salt according to the invention is a salthaving an ammonium ion of the formula R₁ R₂ R₃ NH⁺ wherein each of R₁,R₂, and R₃ is independently hydrogen, methyl, or ethyl. Examples of suchsalts include but are not limited to those salts the cations of whichare NH₄ ⁺, Ca²⁺, Mg²⁺, Na⁺, K⁺, or Li⁺ and the anions are acetate, Cl⁻,NO₃ ⁻, or HSO₄ ⁻ /SO₄ ²⁻. The contaminated soil and leachant are mixedtogether in any convenient manner such as by stirring the contaminatedsoil and the leachant solution together in a container. The ratio ofsoil to leachant will typically be 1 part by weight soil to from 2 to 10parts by weight leachant. Preferably, the ratio of soil to leachant is 1part by weight soil to from 2 to 5 parts by weight leachant. The soilwill be contacted for a time sufficient to transfer at least a portionof the heavy metals and mercury to the leachant. The soil will typicallybe in contact with the leachant for up to 60 minutes. Optionally, at thestart of the contacting period, a coarse solid phase of denser or largersoil particles, which is generally leached more rapidly than the soilfines, is separated by known size separation techniques, such as wetclassification, centrifugal separation, hydrocyclone separation, or wetscreening as set forth above. The mercury removal process can be carriedout at any temperature and will typically be carried out at slightlyelevated (40°-80° C.) temperature. The minimum acid concentration in theleachant solution must be such that a pH of 7 or below is maintainedduring the leaching period in which the soil and/or the soil fines arecontacted by the leachant solution. The maintenance of the pH of theleachant solution can be accomplished by intermittent addition of theacid to the solution or by continuous addition of the acid to thesolution. The method of addition is not critical as long as the pH ismaintained below 7. While the acid concentration can be anyconcentration required to maintain the pH at or below 7, the maximumacid concentration in the leachant solution will typically be about 2moles per liter. In cases where the acid is a weak acid such as aceticacid the preferred concentration is in the range of 1-10 wt % and morepreferably, in the range of 1-5 wt %. In cases where the acid is astrong acid such as hydrochloric acid, the preferred concentration is inthe range of 0.01-1 molar and more preferably in the range of 0.05-0.5molar. The minimum salt concentration in the leachant solution will betypically 0.05% while the maximum salt concentration in the leachantsolution will be about 20% by weight. A preferred salt concentration iswithin the range of 1-20 wt %. A more preferred salt concentration iswithin the range of 2-10%. The concentration of the oxidant in theleachant can range from about 1 eq to about 100 eq and will typically befrom about 2 eq to about 50 eq. The preferred oxidant concentration willbe from about 5 eq to about 25 eq. The oxidant is selected from thegroup consisting of a persulfate salt which includes an alkali metal oralkaline earth metal persulfate salt or, preferably, ammoniumpersulfate; nitric acid, and a halogen preferably bromine. The mercuryremoval process can be carried out in the same manner as that describedpreviously for the removal of heavy metals from soil and may also beaccomplished according to the process flow diagram as set forth in FIG.1.

The process for removing mercury from soil can be modified by theaddition of a cementation step after leaching. The cementation stepcomprises contacting the liquid phase containing the leached, solublemercury species liquid phase with aluminum, iron, or magnesium to removethe soluble mercury species from liquid phase. The resulting processamounts to a complete remediation of mercury from the mercury-containingsoil. Aluminum is the preferred metal for cementation. The amount ofmetal used in the cementation step is from about 1 eq to about 100 eqbased on 1 eq of Hg with the preferred amount being in the range of fromabout 5 eq to about 50 eq. The metal will be in contact with the liquidphase for a period of time from about 1 min to about 5 hr, preferablyfrom about 10 min to about 1 hr. A cementation step can also be added tothe process for removing heavy metals other than mercury from soil orfrom paint chips when the metals removed are reduceable by the cementingmetal, examples of which are copper or lead. In such a case, the liquidphase containing the leached, soluble lead or copper species iscontacted with aluminum, iron, or magnesium to remove the solublemercury species from liquid phase. Aluminum is the preferred metal forcementation. The amount of metal used in the cemetation step is fromabout 1 eq to about 100 eq based on 1 eq metal with the preferred amountbeing in the range of from about 5 eq to about 50 eq. The metal will bein contact with the liquid phase for a period of time from about 1 minto about 5 hr, preferably from about 10 min to about 1 hr.

The process according to the invention which removes lead, copper, ortin from paint chips containing one or more of said metals comprisescontacting the paint chips with an aqueous acid leachant selected fromthe group consisting of formic acid, acetic acid, propionic acid,methanesulfonic acid, hydrochloric acid, nitric acid, and sulfuric acidfor a period of time sufficient to transfer at least a portion of lead,copper, or tin or any combination thereof from the paint chips to one ormore soluble species in the leachant. The process is carried out bycontacting the paint chips with the leachant preferably by mixing theleachant and paint chips together in a vessel for a period of time fromabout 2 hr to about 12 hr. The ratio of leachant to paint chips orresidue containing paint chips can be from about 2 to about 10 and ispreferably from about 3 to about 5. The leaching process can be carriedout at a temperature of from 10° C. to 100° C. or, preferably, from 20°C. to 80° C. Most preferably, the paint chips will be reduced to a fineparticle size by grinding or the like to increase the surface area ofthe chips and hence facilitate the removal of the lead, copper, or tintherefrom. The time required to remove the metal will vary with thenature of the paint chips and the amount of metal in the paint chips andcan be easily determined by those having ordinary skill in the art. Thepaint chips can be contacted repeatedly by the leachant until the metallevel is sufficiently reduced to meet a level set by law or some otherstandard. In one embodiment wherein paint chips containing lead areleached according to the process of the invention, the leaching iscontinued until the lead level is below the TCLP limit (ToxicityCharacteristic Leaching Procedure) as determined by RCRA SW-846, Method1311. The soluble lead or copper species or a combination thereof can beremoved from the solution which remains after leaching by contacting theliquid phase containing the leached, soluble lead or copper species withaluminum, iron, or magnesium to remove the soluble mercury species fromliquid phase.

The following examples are meant to illustrate but not limit theinvention.

EXAMPLE 1 Dissolution of Lead Metal by Various Leachants

Lead granules (0.20-0.30 g, 30 mesh) were stirred in an open 50 mLErlenmeyer flask at 300 rpm with the following leachant solutions (25mL) for 16 hrs, filtered, dried, and the residual lead weighed. Thepercent lead dissolved is listed in Table 1.

                  TABLE 1                                                         ______________________________________                                        Leachant          % Pb Dissolved                                              ______________________________________                                        5% HOAc           32                                                          5% HOAc/5% HCl     9                                                          10% HOAc          29                                                          5% NH.sub.4 OAc   21                                                          5% HOAc/2.5% NaOAc                                                                              36                                                          5% HOAc/2.5% NH.sub.4 OAc                                                                       43                                                          ______________________________________                                    

The residual lead recovered from the acetic acid dissolution was notpassivated by the treatment. Subjecting the residual lead from the 5%HOAc treatment to two additional acetic acid treatments gave a further35% and 32% dissolution. Longer exposure of the lead granules to aceticacid led to more complete dissolution (63 hr, 97% dissolution). Resultsfrom similar experiments with lead and lead oxide are summarized in FIG.6.

EXAMPLE 2 Dissolution of Lead Metal in a Matrix of Soil Constituents

Lead granules (30 mesh), the soil constituent (50 g), and leachant (5%HOAc/2.5% NH₄ OAc) were stirred in open flasks for 66 hr beforefiltering and analyzing the filtrate for dissolved lead by atomicabsorption spectroscopy. The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                  Soil        Leachant                                                Pb in Soil                                                                              Component   Vol (ml) Pb Recovered                                   ______________________________________                                        0.1169 g  sand        250      94%                                            0.1132 g  clay #1     250      21%                                            0.1162 g  clay #2     500      19%                                            ______________________________________                                    

The recovery of lead from the sand matrix was as high as in the absenceof sand (compare with Example 1). In the presence of clay, the recoveryof lead was lower. The lower recovery in the presence of clay isconsistent with the strong metal binding capability of clays. Bycomparison, experiments using acetic acid alone or hydrochloric acidalone as in FIGS. 2 and 3 show virtually no recovery of lead from a claymatrix.

EXAMPLE 3 Repetitive Leaching of a Lead Oxide/Clay Matrix by VariousAcetic Acid Based Leachants

Lead oxide (10 mg), clay (2 g), and leachant (10 mL) were stirred for 1hr, centrifuged, the filtrate removed and analyzed, and fresh leachantadded. The leachings were repeated three times. The results are shown inFIG. 2. The acetic acid/ammonium acetate and acetic acid/calcium acetatecombinations are especially effective. The effectiveness of variouspercentages of acetic acid and ammonium acetate are shown in FIG. 4. Thecombination of acid and salt is surprisingly effective.

Repetition of the experiment with 16, 22, 22, and 22 hr contact timesgave similar results; 5% HOAc removed 12%, 5% HOAc/2.5% NH₄ OAc removed100%, and 5% HOAc/2.5% Ca(OAc)₂ removed 85% of the Pb. Repetition of theexperiment with 30 minute contact times gave results identical to thosewith 60 minutes contact times.

EXAMPLE 4 Repetitive Leaching of a Lead Oxide/Clay Matrix by Various0.1M Hydrochloric Acid Based Leachants

Lead oxide (10 mg), clay (2 g), and leachant (10 mL) were stirred for 1hr, centrifuged, the filtrate removed and analyzed, and fresh leachantadded. The leachings were repeated three times. The results are shown inFIG. 3. The 0.1M HCl/NH₄ Cl and 0.1M HCl/CaCl₂ combinations areespecially effective, but the rate at which Pb is removed is slower thanwith the acetic acid based leachants.

EXAMPLE 5 Repetitive Leaching of Various Metal Oxides in a Clay Matrixby an Acetic Acid/Ammonium Acetate Leachant

The metal oxide (10 mg), montmorillonite clay (2 g), and 5% aceticacid/2.5% ammonium acetate leachant (10 mL) were stirred for 1 hr,centrifuged, the filtrate removed and analyzed by atomic absorption, andfresh leachant added. The leachings were repeated three times. Thecumulative percent metal leached is shown below. Bismuth and zinc areremoved very efficiently.

    ______________________________________                                                    Cumulative % Metal Leached                                                    Leaching Contact                                                  Metal Oxide 1.sup.st                                                                             2.sup.nd   3.sup.rd                                                                           4.sup.th                                   ______________________________________                                        Bi.sub.2 O.sub.3                                                                          62     89         97   99                                         ZnO         55     62         87   89                                         ______________________________________                                    

EXAMPLE 6 Extraction of Lead from Leachant by Carbon

A stock solution of 968 ppm lead was prepared in 5% acetic acid/2.5%ammonium acetate. A series of 6 dram vials were loaded with the stocksolution (15 mL) and carbon (3 g). The vials were mixed by rotation andaliquots removed for analysis. The lead remaining in each startingsolution is tabulated below. Activated carbon efficiently adsorbs leadfrom the acetic acid/ammonium acetate leachant.

    ______________________________________                                                     % Available Lead                                                              adsorbed on C                                                    Carbon        1            4    22 hr                                         ______________________________________                                        Calgon GRC-22                                                                              77           95    96                                            Norit-RO     91           83    89                                            Norit-C      94           94    95                                            ______________________________________                                    

EXAMPLE 7 Acid Stripping of Lead from Carbon

The lead bearing carbon (Calgon GRC-22, 3.00 g) from example 6 wasrotated in a vial with 3M HCl (10 mL) for 3 hrs. The acid solution wasremoved and analyzed by atomic absorption, and fresh hydrochloric acidwas added and stripping continued another 2 hr. The carbon was strippedof 54% of the bound lead.

EXAMPLE 8 Procedure for the Simultaneous Leaching and Extraction of Leadfrom Soil

A sandy loam (3 g) containing lead oxide (10 mg) is sieved through aNo.4 (2.00 mm) sieve and mixed with a particulate activated carbon(Calgon GRC-22, 2 g) in a vial. The 5% acetic acid/2.5% calcium acetateleachant (15 mL) is added, and the slurry mixed by rotation (50 rpm) for2 hr. The carbon particles are removed by sieving through a No. 4 sieve.The isolated carbon particles are washed with water to remove traces ofadhering soil fines, dried in an oven at 105° C. and after microwavedigestion the solution is analyzed by atomic adsorption. The lead isthus removed from the soil matrix and adsorbed onto the carbonparticles.

EXAMPLE 9 Cementation by Aluminum

This example shows the effectiveness of aluminum as a cementation agentfor removing lead and copper from loaded leachants. A leachate solution(100 mL) containing Pb, Cu, Cd, Zn, and Fe in 5% acetic acid/5% sodiumacetate was stirred with crushed aluminum foil pieces (1.00 g) in anopen beaker. Aliquots (5 mL) were removed periodically to monitor thecementation of heavy metal ions by the aluminum, filtered and subjectedto ICP (Inductively Couple Plasmid Spectroscopy) analysis. The results,which are given in Table 3, show that aluminum successfully removed 99%or more of the copper and lead ions by reduction to insoluble elementalcopper and lead.

                  TABLE 3                                                         ______________________________________                                               Metal Concentration (μg/mL)                                                         t = 10  t = 0.5  t = 4                                                                              % Metal                                 Metal  t = 0    min     hr       hr   Removal                                 ______________________________________                                        Cu     56       36      0.4      0.4  >99                                     Pb     826      801     17       11   99                                      Cd     7.5      7.5     7.5      7.9  0                                       Zn     6570     6450    7100     7390 0                                       Fe     68       67      67       69   0                                       ______________________________________                                    

EXAMPLE 10 Leaching of Lead From Paint Chips in Sandblasting Waste atRoom Temperature

Sandblasting waste, sieved to a <2.00 mm fraction, contained sand, paintchips, and fines. The waste (2.00 g) and leachant (10.0 mL) were loadedinto a polyethylene centrifuge tube, capped and shaken for 2 hr at roomtemperature (20°-22° C.). The tube and contents were centrifuged to givea solid pellet and leachant. The leachant was removed by pipette andweighed. The solid pellet was contacted again with fresh leachant. Thiscontacting, centrifuging and replacement with fresh leachant wasrepeated a total of 5 times. The leachates were filtered (<0.45 μm) andanalyzed by ICP for lead. The final treated solids were digested by theEPA acid digestion method and analyzed for lead. The results are shownin Table 4 for three leachants. The results indicate that at roomtemperature after five contacts partial lead removal (58-79%) from thewaste was achieved.

                  TABLE 4                                                         ______________________________________                                        Leaching of Waste Sandblasting Sand (<2.00 mm) with                           Various Leachants                                                             Temperature = 22-25 °C.                                                       Cumulative % Pb                                                               Leached             Initial.sup.1                                                                         Final.sup.2                                       Leaching Contact #  [Pb]    [Pb]                                       Leachant.sup.3                                                                       1       2     3     4   5     (ppm) (ppm)                              ______________________________________                                        1      33      45    51    55  58    3635  1540                               1      34      46    52    56  59    4094  1690                               2      59      71    75    78  79    3913  818                                2      55      68    72    75  76    3721  876                                3      54      69    74    76  78    3855  848                                3      58      71    76    79  80    3433  671                                ______________________________________                                         .sup.1 Based upon the total Pb detected in leachant plus Pb retained in       sand as determined by nitric acid digestion.                                  .sup.2 Based upon nitric acid digestion of treated sand.                      .sup.3 Leachant:                                                              1 = 5% acetic acid/5% calcium acetate                                         2 = 0.1M HCl/5% calcium chloride                                              3 = 0.5M HCl/5% calcium chloride.                                        

EXAMPLE 11 Leaching of Lead From Paint Chips in Sandblasting Waste at60° C.

The procedure described in Example 10 above was repeated at 60° C. Theagitation was accomplished by magnetic stirring while heating in a waterbath. The results are shown in Table 5 and show that the lead removalwas much higher (87-96%) than in the corresponding experiments at roomtemperature.

                  TABLE 5                                                         ______________________________________                                        Leaching of Waste Sandblasting Sand (<2.00 mm) with                           Various Leachants                                                             Temperature = 60° C.                                                   Cumulative % Pb                                                               Leached                Initial.sup.1                                                                          Final.sup.2                                   Leaching Contact #     [Pb]     [Pb]                                          Leachant.sup.4                                                                        1      2      3    4    5    (ppm)  (ppm)                             ______________________________________                                        1       63     76     81   85   89   3376   376                               1       62     73     78   81   87   3226   429                               3       82     91     94   94   95   3785   183                               3       79     91     94   95   95   4267   197                               4       82     91     93   94   96   4646   191                               4       81     91     93   95   96   4354   154                               ______________________________________                                         .sup.1 Based upon the total Pb detected in leachant plus Pb retained in       sand as determined by nitric acid digestion.                                  .sup.2 Based upon nitric acid digestion of treated sand.                      .sup.3 Leachant:                                                              1 = 5% acetic acid/5% calcium acetate                                         3 = 0.5M HCl/5% calcium chloride                                              4 = 20% methanesulfonic acid.                                            

EXAMPLE 12 Leaching of Mercury from Soil

Mercury metal (10-12 mg) was accurately weighed and mixed with cleansoil (2.00 g, Goldridge Sandy Loam, Russian River Valley, Calif.) in a50 mL rounded bottom flask equipped with a magnetic stirrer, condenser,and thermocouple. The mercury amended soil was stirred with 5% aceticacid/5% calcium acetate (10 mL) containing ammonium persulfate (10 and25 eq.) at 60° C. for 1 hr. The resulting soil slurry was quantitativelytransferred to a 50 mL polyethylene centrifuge tube, centrifuged, andthe supernate decanted and analyzed. The remaining soil was leached with5% acetic acid/5% calcium acetate solution (10 mL), centrifuged, and thesupernate decanted five times. All supernates were analyzed for mercuryby ICP, and the remaining soil was acid digested (EPA Method 3050) todetermine the residual mercury concentration in the treated soil. Theresults are shown in Table 6.

                  TABLE 6                                                         ______________________________________                                        Mercury Removal from Soil                                                     Persulfate                                                                            Contact #, Leachate (μg/g Hg)                                                                 Residual Recovered                                 (Eq.)   1*     2      3    4   5   6   Hg (μg/g)                                                                         Hg (%)                          ______________________________________                                        10      316    264    156  44  22  16  1940   65                              25      464    216    203  80  26  12  1010   81                              ______________________________________                                         *Only the first contact contained oxidant plus leachant. Remaining            contacts were done with leachant only.                                   

EXAMPLE 13 Cementation of Mercury by Aluminum

Two solutions of mercuric chloride in (a) 5% acetic acid/5% calciumacetate and (b) 0.1M HCl/5% calcium chloride were prepared,approximately 350 ppm Hg. A portion of each solution (25 mL) was stirredwith crushed aluminum foil pieces (25 mg) under nitrogen. Aliquots (5mL) were removed periodically to monitor the cementation of the mercuricion by the aluminum, filtered and subjected to ICP analysis. The resultsare in Table 3. It is readily apparent that aluminum successfullyremoved the mercuric ion by reduction to insoluble elemental mercury orreduction and amalgation to excess aluminum.

                  TABLE 7                                                         ______________________________________                                        Hg Concentration (μg/mL) % Hg                                              Leachant  t = 0   t = 0.5 hr                                                                             t = 1 hr                                                                            t = 2 hr                                                                             Removal                               ______________________________________                                        HOAc/Ca(OAc).sup.2                                                                      365     311      23    <1     >99                                   HCl/CaCl.sub.2                                                                          359      12      <1    <1     >99                                   ______________________________________                                    

EXAMPLE 14 Electrowinning of Lead-Containing Leachant

The leachate was produced by mixing 300 g of soil (21,000 ppm of Zn;4,500 ppm of Pb) with 3 L of a 5%HOAc/5%NaOAc solution. The mix was puton an Eberbach tableshaker for 1.5 hrs and then allowed to settle overnight. The next day the supernatant was decanted off: analysis indicateda leachate Pb concentration of 215 ppm and a Zn concentration of 1,770ppm. An electrowinning cell with the following materials and dimensionswas used: four aluminum cathodes (2.9"w×3.5"h- total area of 81.2 in² ;five titanium anodes (2.0"w×3.0"h- expanded metal); electrowinning cell-liquid dimensions of 2.6"×3.5×7.0"l, for a total volume of 64 in³ =1054ml; a leachate reservoir of 2L; tubing of 1/4"ID. The plates were placedin alternating fashion with a distance of 1/2"between anodes andcathodes. 2 L of the above leachate was recycled through the system viaa FMI pump and a rate of 120 ml/min. Electric current was established at2.25 V and at which point gas evolution was noted; voltage was thenreduced to 1.75 V and maintained for 5 hrs.

Aliquots were taken every hour and analyzed for Pb:

    ______________________________________                                        Sample      Resulting Pb levels                                               ______________________________________                                        1 hr        216                                                               2 hr        172                                                               3 hr        140                                                               4 hr        118                                                               5 hr        99.5                                                              ______________________________________                                    

EXAMPLE 15

In this example experimental runs were made in relation to leaching oflead from sand fines (fines from a rifle range using a leachantcomprised of a strong mineral acid (hydrochloric acid) plus a weak acid(acetic) in combination with two levels of salt either sodium chlorideor calcium chloride. The summary Table 8 below and Spreadsheet Table 9below show the results. The conditions of the runs were as follows:

Conditions:

soil: Firing range fines found to contain 4-4.5% lead

Leachants:(20 ml volume)

1.) 0.01M Hcl / 1%wt NaCL [0.17M]/ 1%wt HOAc

2.) 0.01M Hcl / 1.9% CaCl₂ [0.17M]/ 1%wt HOAc

3.) 0.01M Hcl / 2.6% NaCl [0.45M]/ 1%wt HOAc

4.) 0.01M Hcl / 5% CaCl₂ [0.45M]/ 1%wt HOAc

Ratio: 1:10 soil to leachant

Contacts: Four consecutive 15 minute contacts at room temperature

Agitation: Wrist Action Shaker, High

                  TABLE 8                                                         ______________________________________                                                    %         Residual   Leached                                      Leachant    Removal   [Pb] (ppm) [Pb] (ppm)                                   ______________________________________                                        1: 0.01M Hcl/                                                                             75/73     9,990/10,900                                                                             29,723/29,378                                1% NaCl/1% HOAc                                                               2: 0.01M Hcl/                                                                             82/80     7,350/8,170                                                                              33,297/33,044                                1.9% CaCl.sub.2 /1% HOAc                                                      3: 0.01M Hcl/                                                                             77/77     8,800/8,800                                                                              30,015/30,229                                2.6% NaCl/1% HOAc                                                             4: 0.01M Hcl/                                                                             86/87     5,560/5,390                                                                              34,888/34,802                                5% CACl.sub.2 /1% HOAc                                                        ______________________________________                                    

From these results the calcium seems to give an advantage over sodiumand a higher concentration of salt may give a slight advantage over alower concentration.

Note: During a Treatability Study (TS), it was also found that aleachant of only strong mineral acid and salt (5% calcium chlorideadjusted to pH 1 with hydrochloric acid) removed 81-83% of the lead infour 15 minute contacts at pH 1 (first contact went to pH 1.4 and cameright down in second contact to pH 1). The concentration of lead reachedin that TS study was 35,474 and 36,050 ppm in duplicate examples, whichresults are very comparable to the result above in this example usingleachant #4 at pH 2.3-3.4.

                                      TABLE 9                                     __________________________________________________________________________    T.S. #                                                                            N/A            Metal: LEAD                                                Date:                                                                             5/16/95        Leachant: VARIOUS                                          Comments/Notes: 4 × 15 Minute Contacts                                                     Leachant Volume: 20 mL                                     __________________________________________________________________________                                Total                                                           Metal in                                                                           Leachate                                                                           Metal                                                                             Metal                                                                             Total                                                                             Metal                                     Sample    Contact                                                                           Leachate                                                                           Weight                                                                             Leached                                                                           Leached                                                                           Metal                                                                             Removed                                   #   pH    #   (ug/mL)                                                                            (g)  (ug)                                                                              (ug)                                                                              (ppm)                                                                             (%)                                       __________________________________________________________________________    11  3.63  1   1460.00                                                                            18.22                                                                              28601                                                                             26801                                                                             13301                                                                             33%                                       12  3.04  2   774.00                                                                             20.35                                                                              15751                                                                             42352                                                                             21176                                                                             53%                                       13  2.7   3   499.00                                                                             20.37                                                                              10165                                                                             52517                                                                             26258                                                                             66%                                       14  2.5   4   339.00                                                                             20.44                                                                               6929                                                                             59446                                                                             29723                                                                             75%                                       15  Residual Metal in soil: 9990                                              Soil wt. (g)                                                                            2.00     Total Metal in Sample (ppm)                                                                39713                                         21  3.53  1   1170.00                                                                            18.38                                                                              21505                                                                             21505                                                                             10752                                                                             27%                                       22  3.19  2   906.00                                                                             20.26                                                                              18356                                                                             39860                                                                             19930                                                                             49%                                       23  2.77  3   559.00                                                                             20.18                                                                              11281                                                                             51141                                                                             25570                                                                             63%                                       24  2.53  4   372.00                                                                             20.47                                                                               7615                                                                             58756                                                                             29378                                                                             73%                                       25  Residual Metal in Soil: 10900                                             Soil wt. (g)                                                                            2.00                  40278                                         31  3.48  1   1730.00                                                                            18.42                                                                              31867                                                                             31867                                                                             15933                                                                             39%                                       32  2.95  2   881.00                                                                             20.42                                                                              17990                                                                             49857                                                                             24928                                                                             61%                                       33  2.61  3   511.00                                                                             20.22                                                                              10332                                                                             60189                                                                             30095                                                                             74%                                       34  2.42  4   310.00                                                                             20.66                                                                               6405                                                                             66594                                                                             33297                                                                             82%                                       35  Residual metal in Soil: 7350                                              Soil wt. (g)                                                                            2.00     Total Metal in Sample (ppm):                                                               40647                                         41  3.49  1   1680.00                                                                            18.35                                                                              30828                                                                             30828                                                                             15414                                                                             37%                                       42  2.98  2   896.00                                                                             20.49                                                                              18359                                                                             49187                                                                             24594                                                                             60%                                       43  2.63  3   517.00                                                                             20.22                                                                              10454                                                                             59641                                                                             29820                                                                             72%                                       44  2.43  4   317.00                                                                             20.34                                                                               6448                                                                             66089                                                                             33044                                                                             80%                                       45  Residual Metal in Soil: 8170                                              Soil wt. (g)                                                                            2.00     Total Metal in Sample (ppm):                                                               41214                                         51  3.68  1   1410.00                                                                            18.8 26508                                                                             26508                                                                             13254                                                                             34%                                       52  3.14  2   816.00                                                                             20.52                                                                              16744                                                                             43252                                                                             21626                                                                             56%                                       53  2.74  3   511.00                                                                             20.24                                                                              10343                                                                             53595                                                                             26797                                                                             69%                                       54  2.52  4   317.00                                                                             20.3  6435                                                                             60030                                                                             30015                                                                             77%                                       55  Residual Metal in Soil: 8800                                              Soil wt. (g)                                                                            2.00     Total Metal In Sample (ppm):                                                               38815                                         61  3.67  1   1400.00                                                                            18.79                                                                              26306                                                                             26306                                                                             13153                                                                             34%                                       62  3.18  2   820.00                                                                             20.36                                                                              16695                                                                             43001                                                                             21501                                                                             55%                                       63  2.78  3   525.00                                                                             20.24                                                                              10626                                                                             53627                                                                             26814                                                                             69%                                       64  2.53  4   339.00                                                                             20.15                                                                               6831                                                                             60458                                                                             30229                                                                             77%                                       65  Residual Metal in Soil: 8800                                              Soil wt. (g)                                                                            2.00     Total Metal in Sample (ppm):                                                               39029                                         71  3.37  1   1940.00                                                                            18.94                                                                              36744                                                                             36744                                                                             18372                                                                             45%                                       72  2.78  2   886.00                                                                             20.89                                                                              18509                                                                             55252                                                                             27626                                                                             68%                                       73  2.56  3   460.00                                                                             20.65                                                                               9499                                                                             64751                                                                             32376                                                                             80%                                       74  2.33  4   244.00                                                                             20.59                                                                               5024                                                                             69775                                                                             34888                                                                             86%                                       75  Residual Metal in Soil: 5560                                              2.8       2.00     Total Metal in Sample (ppm):                                                               40448                                         81  3.37  1   1930.00                                                                            18.98                                                                              36631                                                                             36631                                                                             18316                                                                             46%                                       82  2.88  2   904.00                                                                             20.3 18351                                                                             54983                                                                             27491                                                                             68%                                       83  2.53  3   464.00                                                                             20.48                                                                               9503                                                                             64485                                                                             32243                                                                             80%                                       84  2.33  4   249.00                                                                             20.58                                                                               5119                                                                             69605                                                                             34802                                                                             87%                                       85  Residual Metal in soil: 5390                                              Soil wt. (g)                                                                            2.00     Total Metal In Sample (ppm):                                                               40192                                         __________________________________________________________________________     Note:                                                                         1 and 2 series = 0.01M HCl/1% NaCl/1% HOAc                                    3 and 4 series = 0.01M HCl/1.9% CaCl2/1% HOAc                                 5 and 6 series = 0.01M HCl/2.6% NaCl/1% HOAc                                  7 and 8 series = 0.01M HCl/5% CaCl2/1% HOAc                              

In the foregoing example the leachant systems were employed on sandfines from a firing range which contained lead. The leachant systemherein may also be employed in a sandblasting waste which contains paintchips containing lead as in examples 10 and 11 above. Typically paintsform films of a solid matrix containing pigments and other adjuvants.Because lead containing paints present a hazard, which must be removed,such lead based paints, are typically sandblasted off the substrates towhich they were applied, breaking the paint film into small particlesreferred to as "chips", which are then found admixed with the sand finesemployed for sandblasting the paint from the substrate. Thissandblasting waste mixture which is accordingly comprised of sand andchips or particles of paint, which contain lead in a solid matrix fromthe paint film can be treated with the leachants described above fortreatment of the firing range fines, to leach the lead from thedispersion of sand and paint solids matrix. The lead is leached from thematrix without any dissolution of the paint solids matrix itself.

What is claimed is:
 1. A process for removing lead from sandblastingwastes comprised of sand and lead containing solid paint chips, whichprocess comprises (1) contacting the sand and solid paint chips with aliquid leachant comprised of (a) a strong mineral acid selected from thegroup consisting of hydrochloric acid and sulfuric acid in aconcentration of 0.01M to about 1.0M and (b) from 1 to about 20% byweight of an alkali metal, alkaline earth metal, or ammonium salt havingone or more anions which form water soluble salts with lead, in such amanner so as to disperse at least a part of the sand and solid paintchips in the leachant to form a liquid phase containing dispersed sandand paint solids and for a period of time sufficient to render at leasta portion of the lead from the dispersed solids to a soluble species inthe liquid, and (2) recovering the lead from the liquid phase.
 2. Aprocess as defined in claim 1 in which said salt is selected from thegroup consisting of sodium chloride and calcium chloride.
 3. A processas defined in claim 1 wherein said liquid leachant further comprises (c)from 1 to about 10% by weight of a weak acid.
 4. A process as defined inclaim 3 in which said weak acid is selected from the group consisting ofacetic acid, formic acid, citric acid and propionic acid.
 5. A processas defined in claim 3 wherein the strong mineral acid is hydrochloricacid, the salt is selected from the group consisting of sodium chlorideand calcium chloride and the weak acid is acetic acid.
 6. A process asdefined in claim 1 in which said process further comprises recoveringthe lead by (3) contacting the liquid phase from step (2) with a metalselected from the group consisting of aluminum, iron and magnesium toremove the soluble lead from the liquid phase.
 7. A process as definedin claim 5 wherein the hydrochloric acid concentration is 0.01M, theacetic acid concentration is 1 w/v% and the salt is calcium chloride ina concentration of 0.17M to 0.45M.